Miscellanea as. 501 



On the Vascular System of the Ascidia. By Prof. C. Heller. 

 It is well known that the heart in the Timicata contracts some- 

 times in one and sometimes in the other direction, and that its con- 

 tents are driven sometimes into the dorsal and sometimes into the 

 ventral vessel ; but hitherto no close investigation has been made of 

 the intimate structure of this organ, or of the nature of the vessels 

 springing from it in the Ascidia. 



In the Ascidia the heart forms a cylindrical, elongated, more or 

 less curved tujje, generally placed at the hinder margin of the 

 stomach, and more rarely further forward, near the latter. It is 

 always enclosed by a special, thinly membranous pericardium, with 

 which it is connected towards the dorsal side. The wall of the 

 heart consists of a layer of thin, delicate muscular fibres, which show 

 distinct transverse striation. These muscular fibres are not placed 

 parallel to each other, but form an elegant network, uniting with 

 each other here and there and then again separating, so as to leave 

 small interspaces between them. 



The two large vascular trunks which originate directly from the 

 heart, and of which one runs forward along the dorsal part of the 

 branchial sac above the endostyle, and the other on the ventral part of 

 the branchial sac below the oesophageal channel, also exhibit a similar 

 structure of wall ; in them also the network formed by delicate trans- 

 versely striated muscular fibres may be observed. These two vascular 

 trunks therefore appear to be direct continuations of the heart, from 

 which they cannot be distinguished by their structure ; and they 

 contract like the true heart, although in a less degree. 



As regards the other vessels, those which are distributed in the wall 

 of the branchial sac and in the external envelope are provided with 

 proper walls ; but the course of the blood in the inner mantle seems to 

 be lacunar. The branchial vessels are all furnished with an endothe- 

 lium consisting of small elongated cells. In the walls of the large trans- 

 verse vascular trunks of the branchial net smooth muscular fibres are 

 listinctly observed ; they run in a crooked course and forming meshes 

 .'ith their neighbours towards the projecting longitudinal partition. 

 The vessels which, in the simple Ascidia, run to the outer tunic 

 and then ramify, are also remarkable in their form and structure. 

 They are always double vessels, which only communicate with each 

 other at the extremity of the last ramifications. The blood flows 

 outwards in one vessel and inwards in the other. The largest trunks 

 exhibit in their walls a distinct layer of smooth muscular fibres, both 

 longitudinal and aimular. The latter are by far the more numerous 

 and lie close together, whereas the longitudinal fibres are more scanty. 

 In the further course of these vessels the ainiular muscular fibres 

 alsobecome rarer; and in the finer terminal ramifications the muscular 

 elements are entirely wanting, and the wall consists of a thin mem- 

 brane composed of roundish cells. The Tessels are not connected 

 with the external envelope, but lie quite loose in lacunas. 



The blood circulating in the vessels is frequently coloured ; and its 

 colour is due to that of the blood-corpuscles. Thus Ascidia fumigata is 

 distinguished by the greenish-yellow colour of its blood, and Ascidia 

 mentida and mainillata by a more brownish colour, whilst in some 

 species, e. g. A, intestiualis, the blood appears quite colourless. — 

 Anzeiger der Akad. dcr Wiss. in Wicn, October IH, 1873. 



